Breast-feeding is associated with the suppression of ovarian function, resulting in a variable period of amenorrhea and infertility. Some studies suggest that breast feeding during this period of amenorrhea can provide effective pregnancy prevention. However, no definitive conclusions can be drawn from these studies because they had small numbers of subjects. The World Health Organization studied the risk of pregnancy during lactational amenorrhea relative to infant feeding status.

The analysis was a multinational, prospective, longitudinal study in five developing countries and two developed countries. Participants were women who delivered at term and were planning to breast-feed for at least six months. Participants were also not planning on using any hormonal contraception. The women were asked to keep a daily record of their breast-feeding. These records were reviewed every two weeks along with a sexual history. Return to fertility was defined as the return of a regular menstrual cycle or pregnancy. Breast-feeding status was defined as full, partial or weaned.

Of the 3,422 women who completed the study, 85 became pregnant during the study. Of these 85 women, 21 conceived after the infant had been totally weaned, leaving 64 women who became pregnant during breast feeding. In the first six months, the cumulative pregnancy rate was similar for full and partial breast feeding. Approximately one half of the pregnancies occurred in women who were already having regular menstrual cycles.

The authors concluded that breast-feeding provides fertility protection equivalent to many nonpermanent contraceptive methods. The pregnancy rate did increase over time, particularly after six months' postpartum. This finding can provide women with the option of an effective contraceptive measure while breast feeding.